here she is as lady mary, one of lord grantham's daughters engaged to matthew finally after two seasons of agonizing, will they, won't they? >> lady mary crawly, will you do me the honor of becoming my wife? >> yes. reporter: part of the fun is being on set was trying to figure out what the scenes being shot gave away about season 3. >> i'm sorry, to snatch him away. >> reporter: hmmmm. this at least is obvious. >> looking forward to the wedding? >> what do you think? i'm looking forward to all sorts of things. >> don't make me blush. reporter: in the wardrobe trailer while we were there, what was being picked out for the next day's shooting looked right for a wedding. but whose? >> that's where the hierarchy matters. >> reporter: see that man? his name is alster bruce. his job is historical advisor meaning correctness cop. >> in britain, in the year 1920, there was a way in which somebody who lived in an or it throw karat i can family would expect to be treated and to interact and to treat somebody from below stairs. >> reporter: below stairs, the domain of the servants is actually a s